In hard disk drives, data is written to and read from magnetic recording media, herein called disks, utilizing magnetoresistive (MR) transducers commonly referred to as MR heads. Typically, one or more disks having a thin film of magnetic material coated thereon are rotatably mounted on a spindle. An MR head mounted on an actuator arm is positioned in close proximity to the disk surface to write data to and read data from the disk surface.
During operation of the disk drive, the actuator arm moves the MR head to the desired radial position on the surface of the rotating disk where the MR head electromagnetically writes data to the disk and senses magnetic field signal changes to read data from the disk. Usually, the MR head is integrally mounted in a carrier or support referred to as a slider. The slider generally serves to mechanically support the MR head and any electrical connections between the MR head and the disk drive. The slider is aerodynamically shaped, which allows it to fly over and maintain a uniform distance from the surface of the rotating disk.
Typically, an MR head includes an MR read element to read recorded data from the disk and an inductive write element to write the data to the disk. The read element includes a thin layer of magnetoresistive sensor stripe sandwiched between two magnetic shields. The shields are constructed so that one is just upstream of the sensor stripe and one is just downstream of the sensor stripe. A constant current is passed through the sensor stripe, and the resistance of the magnetoresistive stripe varies in response to a previously recorded magnetic pattern on the disk. In this way, a corresponding varying voltage is detected across the sensor stripe. The magnetic shields help the sensor stripe to focus on a narrow region of the magnetic medium, hence improving the spatial resolution of the read head.
Earlier MR sensors operated on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varied as the square of the cosine of the angle between the magnetization and the direction of sense current flowing through the read element. In this manner, because the magnetic field of the recording media would effect the magnetization direction within the read element, the change in resistance could be monitored to determine the type of external magnetic field applied by the magnetic recording medium. Most current disk drive products utilize a different, more pronounced magnetoresistive effect known as the GMR or spin valve effect. This effect utilizes a layered magnetic sensor that also has a change in resistance based on the application of an external magnetic field.
It is known that pole tip protrusion (PTP) can occur in read/write heads during operation and further that PTP can cause damage or fly-height modulation when/if the read/write head contacts the adjacent storage disk. Pole tip protrusion results from deformation of the slider ABS caused by thermal expansion of the materials in the vicinity of the read/write head. Particularly, due to the variety of different materials in the slider, the expansion is not uniform. For example, the metal of the read/write shields and poles may tend to expand the most. One type of PTP is environmental or thermal PTP (TPTP), which is an isothermal deformation of the slider ABS caused by thermal expansion mismatch of the materials included in the head (e.g., the slider underbody, the undercoat, the GMR shields, the write poles, the write coil and photoresist, and the overcoat). By way of example, there can be in the range of 1 to 6 nm of protrusion when a read/write head is heated from room temperature to 55 degrees C. Another type of PTP is write PTP (WPTP) which is a deformation of the slider ABS during writing where there are strong temperature gradients caused by heat dissipation in the write coil and yoke due to ohmic losses and eddy currents. By way of example, there can be in the range of 1 to 6 nm of protrusion when the write transducer of a read/write head is writing. Thus, the total for PTP can be in the range of 2 to 12 nm when the head is writing at maximum drive operating temperature. It can be appreciated that this is much more than the 1 to 2 nm of pole tip protrusion (PTR) that may occur due to manufacturing, so the net effect is a protrusion.
One of the design challenges relating to significant WPTP comes from the fact that it only occurs when writing. Thus, if one arranges to fly the head at a height that provides an adequate safety margin from touchdowns when writing, then the read sensor will be spaced an undesirable distance from the recording surface during read operations. On the other hand, if one arranges to fly the head at a height that places the read sensor at a more optimal distance from the recording surface when reading, then during writing operations the WPTP may result in contact with the recording surface. Of course, this issue primarily arises because of the low fly heights that arise from the demand for increased areal densities in disk drive systems. When fly heights were greater, WPTP did not tend to me as much of an issue.
It is against this background and a desire to improve on the prior art that the present invention has been developed.